1. Field of the Invention
The present invention relates to a semiconductor power module which mounts a plurality of power semiconductor switches, such as insulated gate bipolar transistors (IGBTs), gate turn-off (GTO) thyristors and the like in a single package, and a large scale module comprising a plurality of the semiconductor power modules. More particularly, the present invention relates to the large scale module suitable for application fields in which various specifications are required as well as a high reliability for a long life. Especially, the present invention pertains to a power converter suitable for motion control of an electric locomotive, in which very severe reliability for a long life is substantially supposed in nature.
2. Description of the Related Art
As known in the art, the high frequency operation of the power converter can reduce the size and weight of the converter. And the power conversion at higher and higher frequencies is desired for power converters used in control systems for driving electric locomotives, since the compactness and the light weight of the converters suitable for installing in a railcar body are required to increase the transportation efficiency. And the high frequency power conversion of the power converters can simultaneously satisfy the comfortableness of passengers in trains. However, the higher reliability, which will guarantee a long life, is also required for the railcar power converters. For example, the reliability, which will guarantee the long life of more than about thirty years, is scheduled to be required for the next generation railcar power converters.
FIG. 1 is a broken sectional view showing an example of an inner structure of a conventional plastic IGBT module used in such a power converter. A plastic side wall 2 is bonded to an edge of a metallic cooling plate 1. A plastic terminal cap 3 covers a top surface of this plastic side wall 2. A copper plate 5, which is directly bonded or silver-brazed to a bottom surface of a ceramic substrate 4, is soldered onto the metallic cooling plate 1 through a solder 6. A copper plate constituting an emitter wiring pattern 71, a collector wiring pattern 72 and a gate wiring pattern 73 is bonded on a top surface of the ceramic substrate 4. A semiconductor chip 8, such as IGBT chip and the like, is soldered to this copper plate 72 through a solder 13.
An emitter electrode pad on the surface of the semiconductor chip 8 is electrically connected to the emitter wiring pattern 71 by aluminum bonding wires 91, and a gate electrode pad is electrically connected to the gate wiring pattern 73 by an aluminum bonding wire 92. In addition, an emitter terminal 10, a collector terminal 11 and a gate terminal 12 which are made of copper are respectively soldered through solders 13 to the emitter wiring pattern 71, the collector wiring pattern 72 and the gate wiring pattern and are erected upwards. Heads of the emitter terminal 10, the collector terminal 11 and the gate terminal 12 are protruding from the outer surface of the terminal cap 3, which supports and fixes the emitter terminal 10, the collector terminal 11 and the gate terminal 12. Moreover, in order to shield the semiconductor chip 8 from outside air, it is filled with a silicon resin 14, and an epoxy resin 15 is filled onto this silicon resin 14.
The semiconductor power modules used to control the system of driving the electric locomotive are required for the high reliability under the severe conditions of higher temperatures and higher humidities. The above mentioned conventional plastic IGBT module has the structure sealed with the silicon resin 14 or the epoxy resin 15. However, since this resin seal is of semi-seal structure, the conventional plastic IGBT module is of an incomplete sealed structure. Thus, the conventional plastic IGBT module is weak in moisture resistance. So, under the environment of the high temperature and the high humidity, water permeates into the module to thereby cause the performance deterioration of the semiconductor chip 8. This is undesirable in view of the reliability of a long time as the semiconductor power module for the electric locomotive. In addition, there may be a possibility that impurities (sodium (Na), chromium (Cr) and the like) will gradually be doped in the silicon resin used for the resin sealing. This impurities will invade the semiconductor chip 8 during the long operation. This results in a problem that the reliability may drop.
Moreover, outer members 2, 3 constituting the semiconductor power module are made of plastic. Thus, they are also weak in mechanical 35 strength. This results in a problem that the explosion-proof durability is substantially null when the semiconductor chip 8 is exploded by a short circuit accident and the like.
Especially, some kinds of the electric locomotives, such as a suburban train, a subway transit car, a streetcar or the like, frequently repeat starts and stops. Thus, the semiconductor power module used therein are expected to have a very high power cycle durability. For example, a semiconductor power module for an railcar in a next generation is planned to have a high power cycle durability of about ten million times, in a junction temperature variation .DELTA.T.sub.j =40.degree. C. and at a case temperature T.sub.c =50.degree. C. However, in the above mentioned conventional plastic IGBT module, the semiconductor chip 8 and the wiring patterns 71, 72 and 73 made of the copper plates are connected to each other through the aluminum bonding wires 91, 92. Hence, the power cycle durability of the conventional semiconductor power module is, for example, only about one hundred thousand times at present. Therefore, it is difficult to satisfy the required power cycle durability for the next generation railcar.
Moreover, the difference of the thermal expansion coefficients between the metallic cooling plate 1 and the ceramic 4 is large, and the railcar frequently repeats the starts and the stops so as to cause severe temperature fluctuations. Then, the crack caused by the thermal stress due to the severe temperature fluctuations is induced in the solder 6. This results in a problem that a Thermal Fatigue Test (TFT) reliability and a Thermal Cycling Test (TCT) reliability of the conventional semiconductor power module are both low and insufficient. On the contrary, the semiconductor power module for the electric railcar in the next generation is planed to have a TFT reliability of about 50 thousands cycles at .DELTA.T.sub.c =70.degree. C. (T.sub.c =25.degree. C. to 95.degree. C.) and a TCT reliability of about 1000 cycles at .DELTA.T.sub.c =165.degree. C. (T.sub.c =-40.degree. C. to 125.degree. C.). However, a present semiconductor power module attains a low TFT reliability of about 5 thousands cycles and a low TCT reliability of about 100 to 300 cycles at the most, under the above mentioned conditions. This causes a problem that the planned specification will not be attained in the next generation.
On the other hand, various power converters, such as a DC-DC converter, a self-excitation inverter, a separate-excitation inverter and the like, are used in the respective control systems for driving the miscellaneous railcars. A rated specifications are variously changed depending on the type of railcar systems. For example, the suburban train requires a large scale module, which comprises a plurality of the semiconductor power modules, having a rated specification of a 800A, 3300 V class or a 1200A, 3300V class. The Japanese high speed train (referred as "the Shinkansen" train in Japanese) requires the large scale modules having a rated specification of a 1200A, 4500V class. "The InterCityExpress (ICE)", the high speed train in Germany and Switzerland or "the Train a Grande Vitesse (TGV)", the high speed train in France requires the large scale modules having rated specification of 1200A, 6500V classes. Then, various large scale modules having diversified rated specifications are required. Also, changes of the specification of the large scale modules often occur depending upon the changes of system designs. However, in the conventional large scale modules assembling plurality of semiconductor power modules, it is not easy to change the maximum current handling capability or the maximum operating voltage. This results in a problem that the rated specifications of the large scale modules can not be changed rapidly and simply.
Thus, a large scale module was desired and required which could easily change the rated specifications, having structure that could be changed rapidly corresponding to the various specifications requested by users with low cost.